Reflective mask and fabricating method thereof

ABSTRACT

A reflective mask includes a substrate, a light absorbing layer over the substrate, a reflective layer over the light absorbing layer, and an absorption pattern over the reflective layer. The reflective layer covers a first portion of the light absorbing layer, and a second portion of the light absorbing layer is free from coverage by the reflective layer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims priority to U.S. Provisional Application Ser.No. 62/750,775, filed Oct. 25, 2018, which is herein incorporated byreference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,an extreme ultraviolet lithography (EUVL) is implemented to meet a needof a higher resolution lithography process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic diagram of an extreme ultraviolet (EUV)lithography system according to some embodiments of the presentdisclosure.

FIG. 2 is a top schematic view of the reflective mask of FIG. 1, inportion or entirety, according to some embodiments of the presentdisclosure.

FIG. 3 is a cross-sectional view of the reflective mask of FIG. 2 takenalong line 3 of FIG. 2.

FIG. 4 is a flowchart of a method for fabricating the reflective mask,according to some embodiments of the present disclosure.

FIG. 5 to FIG. 7 and FIG. 9 to FIG. 18 are cross-sectional views ofdifferent steps of a method of fabricating the reflective mask of FIG.3, according to some embodiments of the present disclosure.

FIGS. 8A, 8B, and 8C are partial views of the light absorbing layer ofFIG. 6, according to some embodiments of the present disclosure.

FIG. 19 is a flowchart of another method for fabricating the reflectivemask, according to some embodiments of the present disclosure.

FIG. 20 to FIG. 25 are cross-sectional views of different steps ofanother method of fabricating the reflective mask of FIG. 3, accordingto some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The advanced lithography process, method, and materials described in thecurrent disclosure can be used in many applications, including fin-typefield effect transistors (FinFETs). For example, the fins may bepatterned to produce a relatively close spacing between features, forwhich some embodiments of the following disclosure are well suited. Inaddition, spacers used in forming fins of FinFETs can be processedaccording to some embodiments of the following disclosure.

FIG. 1 is a schematic diagram of an extreme ultraviolet (EUV)lithography system 100 according to some embodiments of the presentdisclosure. The EUV lithography system 100 includes a radiation source102, condenser optics 106, a mask stage 110, projection optics 112, anda substrate stage 114. However, other configurations and inclusion oromission of the device may be possible. In some embodiments of thepresent disclosure, the EUV lithography system 100 is also referred toas a stepper or a scanner. In some embodiments of the presentdisclosure, the radiation source 102 is configured to provide EUV light104 having a wavelength in the EUV range. For example, the radiationsource 102 may emit the EUV light 104 using carbon dioxide (CO₂) laserproduced tin (Sn) plasma.

The condenser optics 106 includes a multilayer coated collector and aplurality of grazing mirrors and is configured to collect and shape theEUV light 104 and provide a slit of the EUV light 104 to a reflectivemask 200 on the mask stage 110. The EUV light 104 provided andtransmitted to the reflective mask 200 is then reflected by thereflective mask 200 according to design information on the reflectivemask 200. The reflective mask 200 is also referred to as a mask, a photomask, or a reticle. The mask stage 110 includes a plurality of motors,roller guides, and tables; secures the reflective mask 200 on the maskstage 110; and provides the accurate position and movement of thereflective mask 200 in X, Y and Z directions during alignment, focus,leveling and exposure operation in the EUV lithography system 100. Theprojection optics 112 includes a plurality of mirrors, projecting thelight reflected by the reflective mask 200 onto a resist film 118deposited on a wafer 116 secured by the substrate stage 114. Thesubstrate stage 114 includes motors, roller guides, and tables; securesthe wafer 116 on the substrate stage 114; and provides the accurateposition and movement of the wafer 116 in X, Y and Z directions duringalignment, focus, leveling and exposing operation in the EUV lithographysystem 100 so that the image of the reflective mask 200 is transferredonto the resist film 118 in a repetitive fashion (though otherlithography methods are possible). The system 100, or portions thereof,may include additional items, such as a vacuum system and/or a coolingsystem.

The wafer 116 coated with the resist film 118 is loaded on the substratestage 114 for exposure by the light reflected from the reflective mask200. The resist film 118 is also referred to as a photo resist, aresist, or a photo resist film. The resist film 118 includes a positivetone resist or a negative tone resist. The wafer 116 includes a wafersubstrate.

Reference is made to both FIG. 1 and FIG. 2, in which FIG. 2 is a topschematic view of the reflective mask 200 of FIG. 1, in portion orentirety, according to some embodiments of the present disclosure. Thereflective mask 200 in the EUV lithography system 100 includes an imagezone 10 and a frame zone 30. The image zone 10 is formed according tothe integrated circuit (IC) design layout pattern. The image zone 10includes absorptive regions, which absorb light incident thereon, andreflective regions, which reflect light incident thereon. The reflectiveand absorptive regions of the image zone 10 are patterned such thatlight reflected from the reflective regions projects onto the wafer 116and transfers the pattern of the image zone 10 to the resist film 118which is coated on the wafer 116. The pattern of the image zone 10 canbe transferred to multiple fields of the resist film 118 multiple timesusing multiple exposures with the reflective mask 200.

For each exposure process, the EUV lithography system 100 defines aportion of the reflective mask 200 for exposing light thereon. Anexposure slit of the lithography system 100 may define the portion ofthe reflective mask 200 that will be exposed to the EUV light, includingthe image zone 10 and a black border zone 20 adjacent to and surroundingthe image zone 10. The black border zone 20 of the reflective mask 200is in the frame zone 30. The black border zone 20 on the reflective mask200 corresponds to an edge between patterned fields on the resist film118. Given that the black border zone 20 of the reflective mask 200 isexposed to the EUV light during the exposure process, if the blackborder zone 20 undesirably reflects a portion of light to the resistfilm 118, the edge between the patterned fields on the resist film 118receives intended light intensity and extra background reflected lightfrom the black border zone 20. By etching away the reflective multilayer(ML), the black border zone 20 may eliminate EUV light reflectivity, butnot out of band (OoB) light reflectivity such as deep ultraviolet (DUV)light reflectivity. The DUV light projected onto the edge between thepatterned fields on the resist film 118 causes the dose deviation fromthe target and the critical dimension (CD) error. Therefore, the blackborder zone 20 is configured to have no or minimal reflectivity for EUVlight and OoB light such as DUV light and is configured to not image apattern onto the resist film 118.

Reference is made to FIG. 2 and FIG. 3, in which FIG. 3 is across-sectional view taken along line 3 of FIG. 2. The reflective mask200 includes a substrate 210, a light absorbing layer 220 over thesubstrate 210, a reflective multilayer (ML) 230 over the light absorbinglayer 220, and an absorption pattern 240 over the reflective ML 230. Insome embodiments, the light absorbing layer 220 covers an entire topsurface of the substrate 210. A portion of the reflective ML 230 at theblack border zone 20 is removed to expose the light absorbing layer 220.That is, a portion of the light absorbing layer 220 is free fromcoverage by the reflective ML 230 to form the black border zone 20. Theimage zone 10 of the reflective mask 200 is surrounded by the blackborder zone 20. The absorption pattern 240 is formed over the reflectiveML 230. The absorption pattern 240 includes a plurality of absorptiveregions 242 over the reflective ML 230 and a plurality of spaces 244between the absorptive regions 242 to expose the underlying reflectiveML 230. In some embodiments, portions of the reflective ML 230 exposedby the spaces 244 between the absorptive regions 242 serve as reflectiveregions of the image zone 10.

The reflective ML 230 is configured to reflect EUV light. The absorptiveregions 242 are configured to absorb EUV light. Therefore, thereflective mask 200 reflects a pattern of EUV light according to thepattern of the reflective regions of the image zone 10. At the blackborder zone 20, the reflective ML 230 is etched away and replaced withthe light absorbing layer 220 and a filling material 270. The EUVreflectivity at the black border zone 20 is eliminated by the removal ofthe reflective ML 230 and the exposed light absorbing layer 220. Thefilling material 270 protects the light absorbing layer 220 thereunder,and also protects sidewalls of the reflective ML 230, from harshmanufacturing environments.

Referring again to the creation of EUV light by the radiation source 102of FIG. 1, a carbon dioxide laser light is focused on fuel species suchas tin droplets to generate laser produced plasma that emits EUV light.However, OoB light such as DUV light is also emitted by the ionizedplasma as a byproduct, and a portion of this DUV light is inevitablyreflected by the condenser optics 106 and reaches the reflective mask200. The resist film 118 is also sensitive to this DUV light.Undesirably patterning light onto regions on the resist film 118corresponding to edges between fields or dies on the wafer 116 resultsin an unwanted neighboring die effect. Therefore, in order to reduce theneighboring die effect, the black border zone 20 is configured tominimize reflection of EUV light and/or light with other wavelengths soas to not image a pattern onto the resist film 118. In some embodimentsof the present disclosure, the light absorbing layer 220 is added at theblack border zone 20 of the reflective mask 200, and is made of amaterial having a high absorbance and a low reflectance for DUV lightand EUV light. The light absorbing layer 220 at the black border zone 20is free from coverage by the reflective ML 230 for absorbing incidentDUV light and EUV light during lithography, and preventing the same fromundesirably being reflected to the resist film 118.

Reference is made to FIG. 3. In some embodiments, the light absorbinglayer 220 includes a light-absorbing material, such as a black material,to absorb the EUV light and DUV light emitting onto the black borderzone 20 of the reflective mask 200. In some embodiments, the high energyof the EUV light having a short wavelength is converted into heat.However, this heat at the absorptive regions 242 and the black borderzone 20 can overheat the reflective mask 200 during lithography.Overheating may cause distortion and deformation of the reflective mask200, which would lead to a distorted pattern imaged by the reflectivemask 200 onto the resist film 118.

Therefore, in some embodiments, the light absorbing layer 220 isconfigured to convert the EUV light and/or light with other wavelengthsinto heat and is configured to transmit the heat. Furthermore, in someembodiments, the thermal conductivity of the light absorbing layer 220is anisotropic (directionally dependent). For example, in someembodiments, the light absorbing layer 220 has a first thermalconductivity in lateral directions DL which are substantially parallelto a top surface of the substrate 210 and a second thermal conductivityin a vertical direction DV which is substantially perpendicular to thetop surface of the substrate 210, and the first thermal conductivity ofthe light absorbing layer 220 is higher than the second thermalconductivity of the light absorbing layer 220. This anisotropic thermalconductivity of the light absorbing layer 220 allows the light absorbinglayer 220 to absorb energy from the EUV light and/or light with otherwavelengths and transmit the thermal energy in directions substantiallyparallel to the substrate 210 of the reflective mask 200, therebyreducing heating of the substrate 210 and preventing deformation of thereflective mask 200 during lithography. In some embodiments, the lightabsorbing layer 220 includes sp2-hybrid carbon atoms. For example, thelight absorbing layer 220 includes graphene, graphite, carbon nanotubes,or the like. The light absorbing layer 220 including graphene has ahigher thermal conductivity along the plane of the graphene and a lowerthermal conductivity in a direction normal to the plane of the graphene.In some embodiments, the plane of the graphene of the light absorbinglayer 220 is substantially parallel to the lateral directions DL. Thelight absorbing layer 220 including carbon nanotubes has a higherthermal conductivity in the axial direction of the carbon nanotubes anda lower thermal conductivity in the radial direction of the carbonnanotubes. In some embodiments, the axial direction of the carbonnanotubes of the light absorbing layer 220 is substantially parallel toat least one of the lateral directions DL. In some embodiments, elementsfor dissipating heat through conduction, convection or radiation arearranged around the reflective mask 200.

In some embodiments, the light absorbing layer 220 is able to servephotoelectric conversion and is configured to convert the EUV lightand/or light with other wavelengths into electricity. The energy of theEUV light and/or light with other wavelengths can be transmitted anddissipated in a form of the electricity, by the light absorbing layer220. The light absorbing layer 220 includes an electrical conductor andis configured to transmit the electricity. The light absorbing layer 220is electrically connected to and grounded by a grounding unit 300 so asto conduct the electricity converted from the EUV light and/or lightwith other wavelengths out of the reflective mask 200. The lightabsorbing layer 220 is able to convert the energy from the EUV lightand/or light with other wavelengths into thermal and electrical energy,and transmit them out of the reflective mask 200 along the directionssubstantially parallel to the substrate 210. In some embodiments, thelight absorbing layer 220 includes carbon nanotubes. In someembodiments, the carbon nanotubes included in the light absorbing layer220 can be single walled nanotubes (SWNTs). The light absorbing layer220 formed by SWNTs serves a photoelectric conversion function and isable to convert the EUV light and/or light with other wavelengths intoelectricity. In some embodiments, the grounding unit 300 is disposed ona sidewall of the substrate 210.

With insertion of the light absorbing layer 220, undesired photons canbe captured and dissipated in the form of thermal and/or electricalenergy. In some embodiments, the light absorbing layer 220 may also beconfigured to mitigate unwanted charges and/or heat accumulation at anyportion of the reflective mask 200 including but not limited to theblack border zone 20 and thus benefits the wafer printing quality. Insome embodiments, the light absorbing layer 220 covers an entire topsurface of the substrate 210.

Reference is made to FIGS. 2 and 3. The light absorbing layer 220 hasfirst portions 222 disposed under and covered by the reflective ML 230.The light absorbing layer 220 has a second portion 224 that is free fromcoverage by the reflective ML 230. In some embodiments, the fillingmaterial 270 is in the reflective ML 230, at the black border zone 20,and over the second portion 224 of the light absorbing layer 220. Thesecond portion 224 of the light absorbing layer 220 is interposedbetween the filling material 270 and the substrate 210. The fillingmaterial 270 can be a spin-on-glass (SOG) filling material or the like.The filling material 270 protects the second portion 224 of the lightabsorbing layer 220, and also protects the sidewalls of the reflectiveML 230 from harsh manufacturing environments. A top surface of thefilling material 270 has a rectangular frame shape, and a top surface ofthe second portion 224 of the light absorbing layer 220 also has arectangular frame shape. The first portions 222 of the light absorbinglayer 220 and the reflective ML 230 thereon are arranged at the imagezone 10 and the frame zone 30 of the reflective mask 200. The secondportion 224 of the light absorbing layer 220 is arranged at the blackborder zone 20 of the reflective mask 200.

The reflective ML 230 and the absorption pattern 240 together have athickness B greater than about 300 nm, such that sufficient reflectivityof EUV light is achieved by using a sufficient number of film pairs. Anoverall thickness C of the structure disposed over the substrate 210 issubstantially equal to or greater than the sum of a thickness A of thelight absorbing layer 220, and the thickness B of the reflective ML 230and the absorption pattern 240. The thickness A of the light absorbinglayer 220 is less than a thickness T of the substrate 210, to preservematerial cost and prevent unstable structural integrity.

In some embodiments, the second portion 224 of the light absorbing layer220 has been treated by, for example, bombardment, oxidation, or thelike to increase the roughness of the second portion 224 of the lightabsorbing layer 220, such that EUV light reflectivity and OoB lightreflectivity such as DUV light reflectivity of the second portion 224 ofthe light absorbing layer 220 can be lowered. As a result, the roughnessof the second portion 224 of the light absorbing layer 220 is higherthan the roughness of the first portions 222 of the light absorbinglayer 220.

Reference is made to FIG. 4, which is a flowchart of a method offabricating the reflective mask of FIG. 3 according to some embodimentsof the present disclosure, and to FIGS. 5-18, which are cross-sectionalviews of different steps of the method. As shown in FIG. 5, the methodbegins at step S10 by providing the substrate 210. The substrate 210 mayinclude a substrate made of a low thermal expansion material (LTEM),fused silica, or the like. The LTEM material may include TiO₂ doped SiO₂and/or other suitable materials. The LTEM substrate 210 serves tominimize image distortion due to mask heating. In some embodiments, theLTEM substrate 210 includes materials with a low defect level and asmooth surface.

Reference is made to FIG. 4 and FIG. 6. In step S12, a light absorbinglayer 220 is deposited over the top surface of the substrate 210. Insome embodiments, the light absorbing layer 220 has high absorbance andlow reflectance for EUV light and light of OoB wavelength such as DUVlight. In some embodiments, the light absorbing layer 220 includessp2-hybrid carbon atoms. For example, the light absorbing layer 220includes graphene, graphite, carbon nanotubes, or the like.

Reference is made to FIG. 4 and FIG. 7. In step S14, in someembodiments, the light absorbing layer 220 is optionally polished tohave a fine and uniform top surface. The step of polishing lightabsorbing layer 220 is either included or omitted depending on the lightabsorbing layer 220 formation process and result. For example, if thelight absorbing layer 220 includes graphene with sp2-hybrid carbon atomsand naturally has a flat and uniform top surface, then the step ofpolishing light absorbing layer 220 can be omitted.

FIGS. 8A and 8B are partial views of the light absorbing layer 220 ofFIG. 7, according to some embodiments of the present disclosure. In someembodiments, the light absorbing layer 220 a includes graphene (as shownin FIG. 8A). In some embodiments, the light absorbing layer 220 bincludes graphite or stacked layers of graphene (as shown in FIG. 8B).Graphene has high absorbance and low reflectance for EUV light and OoBlight such as DUV light. In some embodiments, the light absorbing layer220 a/220 b has a reflectance of less than about 3% for DUV light, suchthat a limited amount of light is reflected to the photoresist layer,thereby reducing the neighboring die effect. Moreover, graphene has agreater thermal conductivity along the plane of the graphene, and asmaller thermal conductivity in a direction normal to the plane of thegraphene. The graphene can be grown such that the plane of the grapheneis substantially parallel to the top surface of the light absorbinglayer 220 a/220 b. The light absorbing layer 220 a/220 b made ofgraphene has a higher thermal conductivity along its top surface and alower thermal conductivity in a direction normal to its top surface.Therefore, the energy of the EUV light and the OoB light such as the DUVlight emitting to the black border zone 20 of the reflective mask 200(as shown in FIG. 2) is absorbed by the light absorbing layer 220 a/220b and transmitted towards the sides of the reflective mask 200, and nottoward the substrate 210, thereby limiting the overheating of thesubstrate 210.

FIG. 8C is a partial view of the light absorbing layer 220 of FIG. 7,according to some embodiments of the disclosure. In some embodiments,the light absorbing layer 220 c includes carbon nanotubes. Carbonnanotubes are carbon molecules having cylindrical nanostructures, orrolled sheets of graphene. In some embodiments, the carbon nanotubesincluded in the light absorbing layer 220 c can be SWNTs having long andhollow structures with the walls formed by one-atom-thick sheets ofcarbon. When EUV light and OoB light such as DUV light reach the blackborder zone 20 (as shown in FIG. 2), the carbon nanotubes included inthe light absorbing layer 220 c and exposed at the black border zone 20(as shown in FIG. 2) absorb the EUV light and the OoB light such as theDUV light. In some embodiments, the light absorbing layer 220 c has areflectance of less than about 3% for DUV light, such that a limitedamount of light is reflected to the photoresist layer, thereby reducingthe neighboring die effect. In some embodiments, the light absorbinglayer 220 c formed by SWNTs serves a photoelectric conversion functionand is able to convert the EUV light and the OoB light such as the DUVlight into electrical energy. Namely, the light absorbing layer 220 cformed by SWNTs can form p-n junction diodes and is able to convertphoton energy to electrical energy directly. Moreover, the carbonnanotubes can be arranged such that the axial direction of the carbonnanotubes is substantially parallel to the top surface of the substrate210. In some embodiments, a carbon nanotube of the light absorbing layer220 c can have a thermal conductivity greater than about 3000 W·m⁻¹·K⁻¹in its axial direction, and a thermal conductivity less than about 3W·m⁻¹·K⁻¹ in its radial direction. Therefore, the energy of the EUVlight and the OoB light such as the DUV light tends to be absorbed bythe carbon nanotubes and transmitted towards the sides of the reflectivemask 200, and not toward the substrate 210, thereby limiting the heatingof the substrate 210. Furthermore, in some embodiments, the ratio of thethermal conductivity in the axial direction of the carbon nanotube tothe thermal conductivity in the radial direction of the carbon nanotubecan be increased by increasing the aspect ratio (i.e. length to diameterratio) of the carbon nanotube.

Reference is made to FIG. 4 and FIG. 9. Step S16 includes forming thereflective ML 230 over the top surface of the light absorbing layer 220,in which the light absorbing layer 220 is disposed between thereflective ML 230 and the substrate 210. According to Fresnel equations,light reflection will occur when light propagates across the interfacebetween two materials of different refractive indices. The reflective ML230 includes alternating films of materials having different refractiveindexes. The reflected light is larger when the difference of refractiveindices is larger. When EUV light reaches a surface of the topmost filmof the reflective ML 230, or an interface between any two films of thereflective ML 230, a portion of the EUV light is reflected.

In order to increase the total amount of reflected EUV light, a totalnumber of films included in the reflective ML 230 can be increased. Insome embodiments, the films of materials in the reflective ML 230 havealternating indexes. In other words, high refractive films having ahigher refractive index are arranged at every other film, and lowrefractive films having a lower refractive index are arranged at everyother film. EUV light are reflected at low-to-high index interfaces, andat high-to-low index interfaces. The thicknesses of the films are chosensuch that reflections at different interfaces constructively interferewith each other, for the angle of incident EUV light at which thereflective ML 230 is intended to operate. For example, the thicknessesof individual films are chosen such that the path-length differences forreflections from different high-to-low index interfaces are integermultiples of the wavelength of the EUV light. On the other hand, each ofthe path lengths of reflections from the low-to-high index interfacesdiffer from each of the path lengths of reflections from the high-to-lowindex interfaces by an integer multiple of half a wavelength of the EUVlight. Since the EUV light is inverted (phase shifts 180 degrees) whenreflected at the low-to-high index interfaces, but not when reflected atthe high-to-low index interfaces, these reflections are also in phaseand constructively interfere.

In some embodiments, the reflective ML 230 includes a plurality of filmpairs, for example, molybdenum-silicon (Mo/Si) film pairs (e.g., a layerof molybdenum above or below a layer of silicon in each film pair). Thethickness of each film of the reflective ML 230 depends on the EUVwavelength and the incident angle. The thickness and the film pairs ofthe ML 230 can be adjusted to achieve a maximum constructiveinterference of the EUV light reflected at each interface and a minimumabsorption of the EUV light by the reflective ML 230. The reflective ML230 may be selected such that it provides a high reflectivity to aselected radiation type/wavelength.

In some embodiments, a buffer layer is optionally formed over thereflective ML 230. The buffer layer serves as an etching stop layer in asubsequent patterning or a repairing process of an absorption layer,which will be described in detail later. The buffer layer has differentetching characteristics from the absorption layer. The buffer layerincludes ruthenium (Ru), Ru compounds such as RuB and RuSi, or the like.A low temperature deposition process is often chosen for the bufferlayer to prevent inter-diffusion of the reflective ML 230.

Reference is made to FIG. 4 and FIG. 10. Step S18 includes forming theabsorption layer 240′ over the reflective ML 230 or the buffer layer insome embodiments. The absorption layer 240′ absorbs radiation in the EUVwavelength range projected onto the reflective mask 200. The absorptionlayer 240′ includes a single layer or multiple layers from a group ofchromium, chromium oxide, titanium nitride, tantalum nitride, tantalum,titanium, or aluminum-copper, palladium, tantalum boron nitride,aluminum oxide, molybdenum, or other suitable materials. With a properconfiguration of film layers, the absorption layer 240′ will provideprocess flexibility in a subsequent etching process by having differentetch characteristic from the underlying layer, such as the reflective ML230 and the buffer layer.

Reference is made to FIG. 4 and FIG. 11. Step S20 includes etchingportions of the absorption layer 240′ to form an absorption pattern 240over the reflective ML 230. The patterning process includes resistcoating (e.g., spin-on coating), soft baking, target aligning, exposure,post-exposure baking, developing the resist, rinsing, drying (e.g., hardbaking), other suitable processes, and/or combinations thereof.Alternatively, the photolithography exposing process is implemented orreplaced by other proper methods such as maskless photolithography,electron-beam writing, direct-writing, and/or ion-beam writing.

Next, an etching process is followed to remove portions of theabsorption layer 240′ to form the absorption pattern 240. With thepatterned resist layer serves as an etch mask, the underlying layer(e.g. the absorption layer 240′) is etched through the openings of thepatterned resist layer while the portion of the underlying layer coveredby the resist layer remains. The etching process may include dry(plasma) etching, wet etching, and/or other etching methods. Forexample, a dry etching process may implement an oxygen-containing gas, afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), achlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), abromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containinggas, other suitable gases and/or plasmas, and/or combinations thereof.After the etching process, the patterned resist layer may be removed bya suitable technique, such as stripping or ashing.

Reference is made to FIG. 4 and FIG. 12. In step S22, the opening 232 isformed at the black border zone 20 of the reflective mask 200 (as shownin FIG. 2). The opening 232 extends through the absorption pattern 240and the reflective ML 230 to expose a portion of the light absorbinglayer 220. The opening 232 may be formed by performing one or moresuitable etching processes, such as forming a mask layer 250 over theabsorption pattern 240 and the reflective ML 230, patterning the masklayer 250, and performing plural etching processes on the absorptionpattern 240 and the reflective ML 230 using the patterned mask layer 250as an etch mask. The etching process performed on the absorption pattern240 can be similar as that discussed in FIG. 11. The patterned masklayer 250 remains over the absorption pattern 240 and the reflective ML230 after the light absorbing layer 220 is exposed.

Reference is made to FIG. 4 and FIG. 13. In some embodiments, the methodfurther includes step S24, in which the top surface of the exposedportion of the light absorbing layer 220 is roughened by a treatment 260such as oxidation, bombardment, or the like. A greater roughness of thetop surface of the exposed portion of the light absorbing layer 220results in a lower reflection of the EUV light and/or the OoB light suchas the DUV light by the exposed portion of the light absorbing layer 220at the black border zone 20 of the reflective mask 200. The mask layer250 protects the underlying absorption pattern 240 and the reflective ML230 from being damaged by the treatment 260.

Reference is made to FIG. 4 and FIG. 14. In step S26, the opening 232 isfilled with a filling material 270. The filling material 270 covers theexposed portion of the light absorbing layer 220. In some embodiments,the filling material 270 is a transparent, flowable, and low thermalexpansion material. For example, the filling material 270 can be aspin-on-glass (SOG) filling material or the like. The filling material270 is dispensed in the opening 232. The filling material 270 protectsthe light absorbing layer 220 thereunder, and also protects thesidewalls of the reflective ML 230, from harsh manufacturingenvironments in subsequent steps.

Reference is made to FIG. 4 and FIG. 15. In step S28, a first bakingprocess is performed to cure the filling material 270. After the firstbaking process, a solvent of the filling material 270 is removed, andthe filling material 270 becomes solid. The top surface of the fillingmaterial 270 is higher than the topmost surface of the reflective ML 230to protect the entire sidewalls of the reflective ML 230. In someembodiments, the top surface of the filling material 270 issubstantially coplanar with the top surface of the absorption pattern240. In some other embodiments, the top surface of the filling materialis lower than or higher than the top surface of the absorption pattern.

Reference is made to FIG. 4 and FIG. 16. In step S30, the mask layer 250(see FIG. 15) is removed. The mask layer 250 may be removed by asuitable technique, such as stripping, ashing, dry etching, wet etchingor multiple etching processes including both wet etching and dryetching, depending on the material compatibility and desired patternprofile.

Reference is made to FIG. 4 and FIG. 17. In step S32, a second bakingprocess is performed to dry the filling material 270, and thus thefilling material 270 becomes denser after the second baking process.

Reference is made to FIG. 4 and FIG. 18. After above processes, thereflective mask 200 is obtained. In step S34, other resolutionenhancement techniques such as an optical proximity correction (OPC) maybe performed. The reflective mask 200 may undergo a defect repairprocess using a mask repair system. The mask repair system includes asuitable system, such as an e-beam repair system and/or a focused ionbeam (FIB) repair system.

Reference is made to FIG. 19, which is a flowchart of a method offabricating the reflective mask of FIG. 3 according to some embodimentsof the present disclosure, and to FIGS. 20-25, which are cross-sectionalviews of different steps of the method. In some other embodiments, theopening 232 is formed before the absorption layer 240′ is patterned tobecome the absorption pattern 240. Steps S10 to S18 are substantiallythe same as mentioned above, and are not further described herein. Afterthe structure of step S18 (as shown in FIG. 10) having the substrate210, the light absorbing layer 220, the reflective ML 230, and theabsorption layer 240′ is formed, the following steps are performed inorder: step S20′ in which the opening 232 is formed at the black borderzone 20 of the reflective mask 200 (as shown in FIG. 20), step 22′ inwhich the top surface of the exposed portion of the light absorbinglayer 220 is roughened by the treatment 260 such as oxidation,bombardment, or the like (as shown in FIG. 21), step S24′ in which themask layer 250 is removed (as shown in FIG. 22), step S26′ in which theopening 232 is filled with the filling material 270 (as shown in FIG.23), step S28′ in which a first baking process is performed to cure thefilling material 270 (as shown in FIG. 24), and step S30′ in whichportions of the absorption layer 240′ are etched to form the absorptionpattern 240 over the reflective ML (as shown in FIG. 25). Steps S20′,S22′, S24′, S26′, S28′, and S30′ are similar to steps S22, S24, S30,S26, S28, and S20, respectively, and are not further described herein.

Some embodiments of the present disclosure provide a reflective maskhaving a light absorbing layer that has a portion free from coverage bythe reflective ML at a black border zone. The light absorbing layerabsorbs EUV and OoB light such as DUV light, such that unwantedradiation is not reflected to a photoresist layer during lithography.Additionally, the light absorbing layer can convert the absorbed lightinto thermal or electrical energy, and transmit these in a directionsubstantially parallel to the surface of the reflective mask substrate,such that the reflective mask does not overheat and become distorted.

According to some embodiments of the disclosure, a reflective maskincludes a substrate, a light absorbing layer over the substrate, areflective layer over the light absorbing layer, and an absorptionpattern over the reflective layer. The reflective layer covers a firstportion of the light absorbing layer, and a second portion of the lightabsorbing layer is free from coverage by the reflective layer.

According to some embodiments of the disclosure, a reflective maskincludes a substrate, a reflective layer over the substrate, a fillingmaterial in the reflective layer, a light absorbing layer interposedbetween the filling material and the substrate, and an absorptionpattern over the reflective layer.

According to some embodiments of the disclosure, a method includesforming a light absorbing layer over a substrate. A reflective layer isformed over the light absorbing layer. An absorption pattern is formedover the reflective layer, and the reflective layer is etched to form anopening in the reflective layer to expose a portion of the lightabsorbing layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method, comprising: forming a light absorbinglayer over a substrate; forming a reflective layer over the lightabsorbing layer; forming an absorption pattern over the reflectivelayer; and etching an opening in the reflective layer to expose aportion of the light absorbing layer.
 2. The method of claim 1, furthercomprising: roughening the exposed portion of the light absorbing layer.3. The method of claim 2, further comprising: forming a mask layer overthe reflective layer; patterning the mask layer, wherein etching theopening in the reflective layer is performed using the patterned masklayer as an etch mask; and removing the patterned mask layer afterroughening the exposed portion of the light absorbing layer.
 4. Themethod of claim 1, further comprising: filling the opening with afilling material.
 5. The method of claim 1, wherein forming theabsorption pattern over the reflective layer comprises: forming anabsorption layer over the reflective layer; and etching the absorptionlayer to form the absorption pattern over the reflective layer, whereinetching the absorption layer to form the absorption pattern over thereflective layer is performed prior to etching the opening in thereflective layer.
 6. The method of claim 1, wherein forming theabsorption pattern over the reflective layer comprises: forming anabsorption layer over the reflective layer; and etching the absorptionlayer to form the absorption pattern over the reflective layer, whereinetching the absorption layer to form the absorption pattern over thereflective layer is performed after etching the opening in thereflective layer.
 7. A method, comprising: forming a light absorbinglayer over a substrate; forming a reflective layer over the lightabsorbing layer; forming an absorption pattern over the reflectivelayer; etching an opening in the reflective layer such that a firstportion of the light absorbing layer is covered by the reflective layerand a second portion of the light absorbing layer is exposed through theopening; and performing a roughening treatment on the light absorbinglayer such that a roughness of the second portion of the light absorbinglayer is higher than a roughness of the first portion of the lightabsorbing layer.
 8. The method of claim 7, wherein the absorptionpattern is formed prior to etching the opening in the reflective layer.9. The method of claim 7, further comprising: filling the opening with afilling material; and performing a first baking process to cure thefilling material.
 10. The method of claim 9, further comprising:performing a second baking process to dry the filling material.
 11. Themethod of claim 9, wherein filling the opening with the filling materialis performed such that a top surface of the filling material is higherthan a topmost surface of the reflective layer.
 12. The method of claim7, wherein the light absorbing layer has a first thermal conductivity ina first direction substantially parallel to a top surface of thesubstrate and a second thermal conductivity in a second directionsubstantially perpendicular to the top surface of the substrate, and thefirst thermal conductivity is higher than the second thermalconductivity.
 13. The method of claim 7, further comprising: polishingthe light absorbing layer prior to forming the reflective layer.
 14. Amethod of forming a reflective mask, comprising: forming a lightabsorbing layer over a substrate, wherein the light absorbing layer isin contact with the substrate; forming a reflective layer over the lightabsorbing layer; forming an absorption pattern over the reflectivelayer; etching an opening in the reflective layer such that a firstportion of the light absorbing layer is covered by the reflective layerand a second portion of the light absorbing layer is exposed through theopening, wherein the first portion of the light absorbing layer is in ablack border zone of the reflective mask, and the second portion of thelight absorbing layer is in a image zone of the reflective mask, whereinthe image zone is surrounded by the black border zone; and filling theopening with a filling material.
 15. The method of claim 14, whereinfilling the opening with the filling material is performed such that thesecond portion of the light absorbing layer is in contact with thefilling material.
 16. The method of claim 14, wherein filling theopening with the filling material is performed such that the firstportion of the light absorbing layer is free of the filling material.17. The method of claim 14, further comprising: performing a rougheningtreatment on the second portion of the light absorbing layer such that atop surface of the second portion of the light absorbing layer is higherthan a bottom surface of the reflective layer.
 18. The method of claim14, wherein a thickness of the light absorbing layer is smaller than athickness of the substrate.
 19. The method of claim 14, wherein thelight absorbing layer comprises sp2-hybrid carbon atoms.
 20. The methodof claim 14, wherein the light absorbing layer has a reflectance of lessthan about 3% for DUV light.